In a ship with engine-driven propellers for generating the propulsion, and a rudder facility for determining the direction of the propulsion of the ship, the relationship between the initiation and the result of the change in the criteria for the motion of the ship can be recognized without much difficulty by the ship operator. If a change in the direction is occasioned by an adjusting lever or, in the case of older installations, by a handwheel, then the change in the adjusting lever and the change in the direction of travel always correspond to one another. If the ship operator changes the lever position in order to change the direction of travel to starboard, the result of adjusting the lever can be anticipated readily. An adjustment of the lever, which does not lead to the desired result, is practically inconceivable.
On the other hand, if rudder propellers or pump jets are used to propel or steer a ship, the relationships are much more complicated. Particularly when several rudder propellers or pump jets, which can be adjusted individually and independently of one another, are used. The effect of adjusting the rudder propellers or pump jets is much more difficult to anticipate and possible only for an experienced ship operator. This is due to owing to the fact that almost any change in the propulsion direction of the ship is achievable by means of the adjusting motions of the driving units, which is, after all, the main advantage of such propulsion facilities. This difficulty originates particularly from the fact that, with respect to the number of revolutions and the ejection direction of the water jet produced, some adjusting motions of several rudder propellers or pump jets lead to changes in the motion of the ship, which are different from those that are to be expected without detailed knowledge. As a consequence of this, only well-trained ship operators, who practice constantly, can be used for such ships. Even these operators can make steering mistakes, if they are faced with a different situation, for which they are not prepared.